U.S. patent application number 13/208731 was filed with the patent office on 2012-03-08 for nonaqueous electrolyte secondary battery.
Invention is credited to Hiroki INAGAKI, Takuya IWASAKI, Takashi KISHI, Norio TAKAMI.
Application Number | 20120058379 13/208731 |
Document ID | / |
Family ID | 45770958 |
Filed Date | 2012-03-08 |
United States Patent
Application |
20120058379 |
Kind Code |
A1 |
KISHI; Takashi ; et
al. |
March 8, 2012 |
NONAQUEOUS ELECTROLYTE SECONDARY BATTERY
Abstract
According to one embodiment, a nonaqueous electrolyte secondary
battery includes a nonaqueous electrolytic solution, a positive
electrode and a negative electrode is provided. The nonaqueous
electrolytic solution comprises a nonaqueous solvent. The
nonaqueous solvent comprises from 50 to 95% by volume of a
sulfone-based compound represented by the following formula 1:
##STR00001## wherein R.sub.1 and R.sub.2 are each an alkyl group
having 1 to 6 carbon atoms and satisfy R.sub.1.noteq.R.sub.2. The
positive electrode comprises a composite oxide represented by
Li.sub.1-xMn.sub.1.5-yNi.sub.0.5-zM.sub.y+zO.sub.4. The negative
electrode comprises a negative electrode active material being
capable of absorbing and releasing lithium at 1 V or more based on
a metallic lithium potential.
Inventors: |
KISHI; Takashi;
(Yokosuka-shi, JP) ; TAKAMI; Norio; (Yokohama-shi,
JP) ; IWASAKI; Takuya; (Uenohara-shi, JP) ;
INAGAKI; Hiroki; (Kawasaki-shi, JP) |
Family ID: |
45770958 |
Appl. No.: |
13/208731 |
Filed: |
August 12, 2011 |
Current U.S.
Class: |
429/149 ;
320/137; 429/329; 429/330; 429/340; 429/341 |
Current CPC
Class: |
H01M 4/485 20130101;
H01M 10/052 20130101; H01M 2300/004 20130101; Y02T 10/70 20130101;
H01M 10/0568 20130101; H01M 4/505 20130101; Y02E 60/10 20130101;
H01M 4/52 20130101; H01M 10/0569 20130101 |
Class at
Publication: |
429/149 ;
429/340; 429/341; 429/329; 429/330; 320/137 |
International
Class: |
H01M 10/056 20100101
H01M010/056; H02J 7/00 20060101 H02J007/00; H01M 2/10 20060101
H01M002/10 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 7, 2010 |
JP |
2010-200041 |
Claims
1. A nonaqueous electrolyte secondary battery, comprising: a case;
a nonaqueous electrolytic solution provided in the case and
comprising a nonaqueous solvent and a lithium salt dissolved in the
nonaqueous solvent, the nonaqueous solvent comprising from 50 to
95% by volume of a sulfone-based compound represented by the
following formula 1 ##STR00003## wherein R.sub.1 and R.sub.2 are
each an alkyl group having 1 to 6 carbon atoms and satisfy
R.sub.1.noteq.R.sub.2; a positive electrode provided in the case
and comprising a composite oxide represented by
L.sub.1-xMn.sub.1.5-yNi.sub.0.5-zM.sub.y+zO.sub.4, wherein
0.ltoreq.x.ltoreq.1, 0.ltoreq.y+z.ltoreq.0.15, and M is at least
one selected from the group consisting of Mg, Al, Ti, Fe, Co, Ni,
Cu, Zn, Ga, Nb, Sn, Zr and Ta; and a negative electrode provided in
the case and comprising a negative electrode active material being
capable of absorbing and releasing lithium at 1 V or more based on
a metallic lithium potential.
2. The battery according to claim 1, wherein the alkyl group
comprises at least one selected from the group consisting of a
methyl group, an ethyl group, a butyl group and an isopropyl
group.
3. The battery according to claim 1, wherein the sulfone-based
compound comprises at least one selected from the group consisting
of isopropyl methyl sulfone, ethyl isopropyl sulfone, normal butyl
normal propyl sulfone and ethyl normal propyl sulfone.
4. The battery according to claim 1, wherein the negative electrode
active material comprises an oxide comprising Ti.
5. The battery according to claim 4, wherein the oxide comprising
Ti comprises at least one selected from the group consisting of
spinel-type lithium titanate and monoclinic system titanium
dioxide.
6. The battery according to claim 1, wherein the nonaqueous solvent
further comprises a sultone-based compound.
7. The battery according to claim 6, wherein the sultone-based
compound comprises at least one selected from the group consisting
of 1,3-propanesultone and 1,4-butanesultone.
8. The battery according to claim 6, wherein the total amount of
the sulfone-based compound and the sultone-based compound is from
80 to 100% by volume of the nonaqueous solvent.
9. The battery according to claim 1, wherein the nonaqueous solvent
further comprises a cyclic carbonate.
10. The battery according to claim 9, wherein the nonaqueous
solvent comprises the sulfone-based compound by from 50 to 95% by
volume and the cyclic carbonate by from 5 to 50% by volume.
11. The battery according to claim 9, wherein the nonaqueous
solvent comprises the sulfone-based compound by from 50 to 80% by
volume and the cyclic carbonate by from 20 to 50% by volume.
12. The battery according to claim 1, wherein the lithium salt
comprises at least one of LiPF.sub.6 and LiBF.sub.4.
13. The battery according to claim 1, which has been subjected to a
treatment comprising charging the battery at least one or more time
and removing gas in the case.
14. The battery according to claim 1, wherein the negative
electrode active material comprises at least one selected from the
group consisting of spinel-type lithium titanate and monoclinic
system titanium dioxide, the sulfone-based compound comprises at
least one selected from the group consisting of isopropyl methyl
sulfone and ethyl isopropyl sulfone, and the nonaqueous solvent
further comprises a sultone-based compound comprising at least one
selected from the group consisting of 1,3-propanesultone and
1,4-butanesultone, and a total amount of the sulfone-based compound
and the sultone-based compound is 80% by volume or more of the
nonaqueous solvent.
15. The battery according to claim 14, wherein the nonaqueous
solvent further comprises a cyclic carbonate.
16. A battery pack comprising the nonaqueous electrolyte secondary
battery according to claim 1.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is based upon and claims the benefit of
priority from prior Japanese Patent Application No. 2010-200041,
filed Sep. 7, 2010, the entire contents of which are incorporated
herein by reference.
FIELD
[0002] Embodiments described herein relate generally to a
nonaqueous electrolyte secondary battery and a battery pack
comprising the nonaqueous electrolyte battery.
BACKGROUND
[0003] Among secondary batteries, a nonaqueous electrolyte
secondary battery is a secondary battery that charges and
discharges by transfer of lithium ion between positive and negative
electrodes. Since a nonaqueous electrolyte secondary battery uses
an organic solvent as an electrolytic solution, it may provide a
larger voltage than a nickel-cadmium secondary battery and a nickel
metal hydride secondary battery, both of which use an aqueous
solution. A nonaqueous electrolyte secondary battery that is
currently put into practical use uses a lithium-containing cobalt
composite oxide or a lithium-containing nickel composite oxide as a
positive electrode active material, a carbon-based material and
lithium titanate or the like as a negative electrode active
material, and uses a lithium salt such as LiPF.sub.6 and LiBF.sub.4
in the form of a solution in an organic solvent such as cyclic
carbonates and linear carbonates as an electrolytic solution. The
positive electrode active material has an average working potential
of about from 3.4 to 3.8 V based on a metallic lithium potential,
and the maximum potential during charging of from 4.1 to 4.3 V
based on the metallic lithium potential. On the other hand, the
carbon-based material and lithium titanate that are negative
electrode active materials have average working potentials of about
from 0.05 to 0.5 V and 1.55 V, respectively, vs a metallic lithium
potential. By combining these positive and negative electrode
active materials, the battery voltage becomes from 2.2 to 3.8 V,
and the maximum charge voltage becomes from 2.7 to 4.3 V.
[0004] In order to further improve a capacity, use of
LiMn.sub.1.5Ni.sub.0.5O.sub.4 that provides the maximum potential
during charging of from 4.4 to 5.0 V for a positive electrode is
considered. However, in a positive electrode comprising
LiMn.sub.1.5Ni.sub.0.5O.sub.4, a carbonate-based solvent causes an
oxidation reaction during charging, whereby causes deterioration of
cycle performance and generation of gas. Furthermore, sultone and
sulfone-based compounds have high viscosity, and the potential of
oxidative decomposition is increased, but reactivity with a solvent
in a negative electrode is increased. Thus, excellent cycle
performance may not be obtained.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] FIG. 1 is a schematic view showing the cross-section of a
nonaqueous electrolyte secondary battery of a first embodiment;
[0006] FIG. 2 is an enlarged cross-sectional view showing a portion
A of FIG. 1;
[0007] FIG. 3 is an exploded perspective view of a battery pack of
a third embodiment;
[0008] FIG. 4 is a block drawing showing an electrical circuit of
the battery pack of FIG. 3; and
[0009] FIG. 5 is a graph showing the changes in the discharge
capacity maintenance rates in Examples and Comparative
Examples.
DETAILED DESCRIPTION
First Embodiment
[0010] In general, according to one embodiment, a nonaqueous
electrolyte secondary battery of a first embodiment comprises a
case, a nonaqueous electrolytic solution that is housed in the
case, a positive electrode that is housed in the case, and a
negative electrode that is housed in the case. The nonaqueous
electrolytic solution comprises a nonaqueous solvent and a lithium
salt dissolved in the nonaqueous solvent. The nonaqueous solvent
comprises from 50 to 95% by volume of a sulfone-based compound
represented by the following formula 1:
##STR00002##
wherein R.sub.1 and R.sub.2 are each an alkyl group having 1 to 6
carbon atoms and satisfy R.sub.1.noteq.R.sub.2.
[0011] The positive electrode comprises a composite oxide
represented by the following formula (2):
Li.sub.1-xMn.sub.1.5-yNi.sub.0.5-zM.sub.y+zO.sub.4 (2)
wherein 0.ltoreq.x.ltoreq.1, 0.ltoreq.y+z.ltoreq.0.15, and M is at
least one kind of element selected from the group consisting of Mg,
Al, Ti, Fe, Co, Ni, Cu, Zn, Ga, Nb, Sn, Zr and Ta.
[0012] The negative electrode comprises a negative electrode active
material being capable of absorbing and releasing lithium at 1 V or
more based on a metallic lithium potential.
[0013] The positive electrode comprising the composite oxide
represented by Formula 2 may be used at a high potential that is
about from 4.4 to 4.9 V of the maximum potential during charging,
whereas it has high reactivity with the electrolytic solution. By
using the nonaqueous electrolyte solution comprising a nonaqueous
solvent comprising the sulfone-based compound represented by
Formula 1 by from 50 to 95% by volume and a lithium salt dissolved
in the nonaqueous solvent, a high charge capacity may be obtained,
and generation of gas may be decreased. As a result, enlarging of
the interelectrode distance between the positive electrode and
negative electrode in accordance with the progress of
charge-discharge cycles may be suppressed, and thus increase in the
internal resistance in accordance with the progress of the
charge-discharge cycles may be suppressed. Therefore, battery
properties and cycle performance may be improved. Although a
specific mechanism of improvement of the properties is unclear, it
is presumed that the properties are improved since the
sulfone-based compound represented by Formula 1 is anodic
stability, and that the nonaqueous electrolytic solution comprising
the compound forms layers showing improved properties on the
surfaces of the positive and negative electrodes.
[0014] Therefore, by using a negative electrode comprising a
negative electrode active material being capable of absorbing and
releasing lithium at 1 V or more based on a metallic lithium
potential together with the positive electrode and nonaqueous
electrolytic solution, a 3-V class nonaqueous electrolyte secondary
battery having high capacity and a long cycle life may be
realized.
[0015] Hereinafter, the positive electrode, negative electrode,
nonaqueous electrolytic solution and case are explained.
(Positive Electrode)
[0016] The positive electrode comprises a positive electrode active
material, and may further comprise a substance having electron
conductivity such as carbon (hereinafter referred to as an electron
conductive substance) and a binder. A sheet obtained by adding a
binder to a positive electrode active material and an electron
conductive substance, and kneading and rolling the mixture to give
a sheet may be used as the positive electrode. Alternatively, it is
also possible to form a positive electrode material layer on a
current collector by dissolving or suspending a mixture comprising
the positive electrode active material, electron conductive
substance and binder in a solvent such as toluene and
N-methylpyrrolidone (NMP) to give a slurry, applying the slurry on
a current collector, drying and pressing the positive electrode
material layer and current collector to give a sheet.
[0017] Among the composite oxides represented by Formula 2,
LiMn.sub.1.5Ni.sub.0.5O.sub.4 may improve the cycle performance of
the nonaqueous electrolyte secondary battery, and may decrease the
production cost. Furthermore, by replacing a part of Mn or a part
of Ni, or a part of Mn and a part of Ni in Formula 2 with Mg, Al,
Ti, Fe, Co, Ni, Cu, Zn, Ga, Nb, Sn, Zr or Ta, the surface activity
of the positive electrode active material is decreased, whereby
increase in the battery resistance may further be suppressed.
Substitution with Mg or Zr, or by both elements is desirable since
it is highly effective. The amount of substitution (y+z) is
desirably 0.01 or more in view of suppression of surface activity,
and is desirably 0.15 or less in view of improvement of capacity. A
further preferable range is from 0.03 to 0.1. In addition, the
molar ratio of Li (1-x) may vary in the range of
0.ltoreq.x.ltoreq.1 depending on absorption and release of lithium
in accordance with a charge-discharge reaction.
[0018] Among the composite oxides represented by Formula 2, one
kind may be used as the positive electrode active material, or a
mixture of two or more kinds may be used as the positive electrode
active material.
[0019] Examples of the binder may include polytetrafluoroethylene
(PTFE), polyvinylidene fluoride (PVdF), ethylene-propylene-diene
copolymer, styrene-butadiene rubber and the like.
[0020] It is desirable that the positive electrode comprises a
current collector, and a positive electrode material layer formed
on one surface or both surfaces of the current collector. When the
positive electrode material layer comprises the positive electrode
active material, electron conductive substance and binder, the
mixing ratio of the positive electrode active material, electron
conductive substance and binder is preferably from 75 to 98% by
weight of the positive electrode active material, from 1 to 20% by
weight of the electron conductive substance, and from 1 to 7% by
weight of the binder with respect to 100% by weight of the positive
electrode material layer.
[0021] For example, a substrate having electron conductivity such
as a metal or the like may be used for the current collector.
Examples of the current collector may include metal foils, thin
plates or meshes, metal meshs and the like. Examples of the metal
for the current collector may include aluminum, stainless, titanium
and the like.
[0022] It is desirable that the maximum positive electrode
potential during charging falls within the range of from 4.4 to 4.9
V based on a metallic lithium potential. By charging to such
potential, high capacity may be obtained. Furthermore, it is more
desirable to adjust the maximum potential to from 4.6 to 4.8 V
based on a metallic lithium potential since high capacity and
suppression of side reactions may be achieved. In addition, it is
desirable to adjust the minimum potential during discharging of the
positive electrode to from 4.0 to 4.3 V based on a metallic lithium
potential since deterioration of the positive electrode active
material may be minimized and higher cycle performance may be
obtained.
(Negative Electrode)
[0023] The negative electrode comprises a negative electrode active
material, and may further comprise a conductive material, a binder
and the like. The form of the negative electrode may be a pellet
shape, a thin plate shape or a sheet shape.
[0024] The negative electrode is obtained, for example, by adding
the binder to the negative electrode active material and conductive
material, and molding the mixture into a pellet or sheet by
kneading and rolling. Alternatively, a current collector on which a
negative electrode material layer is formed, which is obtained by
dissolving or suspending a mixture comprising the negative
electrode active material, conductive material and binder in a
solvent such as water and N-methylpyrrolidone (NMP) to prepare a
slurry, applying the slurry on the current collector, drying and
pressing to give a sheet, may be used as the negative
electrode.
[0025] It is desirable that the upper limit of the lithium
absorption release potential is adjusted to 3.0 V based on a
metallic lithium potential. Furthermore, examples of the negative
electrode active material being capable of absorbing and releasing
lithium at 1 V or more based on a metallic lithium potential may
include spinel-type lithium titanate, monoclinic system titanium
dioxide, iron sulfide and the like. The kind of negative electrode
active material used may be one kind or two or more kinds. An oxide
comprising Ti such as spinel-type lithium titanate and monoclinic
system titanium dioxide is desirable in that flatness of a
charge-discharge curve is high, the potentials of the positive and
negative electrodes are easily controlled when combined with the
positive electrode, and variation due to progression of cycles is
suppressed to the minimum, whereby high cycle performance may be
realized.
[0026] Examples of the conductive material may include substances
having electron conductivity (hereinafter referred to as electron
conductive substances) such as carbon and metals. The form of the
conductive material is desirably a form of granular, fiber or the
like.
[0027] Examples of the binder may include polytetrafluoroethylene
(PTFE), polyvinylidene fluoride (PVdF), styrene-butadiene rubber,
carboxymethylcellulose (CMC) and the like.
[0028] It is desirable that the negative electrode comprises a
current collector and a negative electrode material layer formed on
one surface or both surfaces of the current collector. When the
negative electrode material layer comprises the negative electrode
active material, conductive material and binder, the mixing ratio
of the negative electrode active material, conductive material and
binder is preferably in the ranges of 73 to 98% by weight of the
negative electrode active material, 1 to 20% by weight of the
conductive material and 1 to 7% by weight of the binder based on
100% by weight of the negative electrode material layer.
[0029] Examples of the current collector may include metal foils,
thin plates or meshes, metal meshes and the like. Examples of the
metal may include copper, stainless, nickel and the like.
[0030] It is desirable that the minimum negative electrode
potential during charging falls within the range of 1 V or more
based on a metallic lithium potential. Accordingly, the side
reaction between the negative electrode and sulfone-based compound
may be suppressed. By adjusting the minimum negative electrode
potential during charging to from 1 to 1.5 V, when the negative
electrode and positive electrode are combined, the side reaction in
which the electrolytic solution is involved may be suppressed to
the minimum, whereby high charge-discharge efficiency and high
cycle performance may be realized. Furthermore, it is more
desirable to adjust the minimum negative electrode potential during
charging to from 1.35 to 1.45 V based on a metallic lithium
potential, deterioration of the negative electrode active material
during cycles may further be decreased, and increase in impedance
may be suppressed. Furthermore, it is more desirable to adjust the
maximum negative electrode potential during discharging to from 1.6
to 2 V based on a metallic lithium potential since deterioration of
the negative electrode active material during cycles may further be
decreased. Examples of the negative electrode active material that
may be operated at such potential may include spinel-type lithium
titanate, monoclinic system titanium dioxide, iron sulfide and the
like. Of these, oxides comprising Ti such as spinel-type lithium
titanate and monoclinic system titanium dioxide provide high
flatness of a charge-discharge curve. Therefore, the negative
electrode comprising the oxide comprising Ti is desirable since it
may readily control the potentials of the positive and negative
electrodes when combined with the positive electrode, may suppress
variation due to progression of cycles and may realize a high cycle
performance.
(Nonaqueous Electrolytic Solution)
[0031] The nonaqueous electrolytic solution comprises a nonaqueous
solvent comprising from 50 to 95% by volume of a sulfone-based
compound represented by Formula 1, and a lithium salt dissolved in
the nonaqueous solvent.
[0032] R.sub.1 and R.sub.2 in Formula 1 are each an alkyl group
having 1 to 6 carbon atoms and satisfy R.sub.1.noteq.R.sub.2.
Examples of the alkyl group may include a methyl group, an ethyl
group, a butyl group, an isopropyl group and the like. When the
kind of alkyl group for R.sub.1 and the kind of alkyl group for
R.sub.2 are the same, the sulfone-based compound has symmetry.
Therefore, the sulfone-based compound is readily precipitated as a
solid in the nonaqueous electrolytic solution, whereby the low
temperature performance of the battery is deteriorated. When the
kind of alkyl group for R.sub.1 and the kind of alkyl group for
R.sub.2 are different, precipitation of the sulfone-based compound
in the nonaqueous electrolytic solution may be suppressed, whereby
excellent low temperature performance may be obtained.
[0033] Preferable Examples of the sulfone-based compound may
include isopropyl methyl sulfone wherein R.sub.1 is a methyl group
and R.sub.2 is an isopropyl group, ethyl isopropyl sulfone wherein
R.sub.1 is an ethyl group and R.sub.2 is an isopropyl group, and
the like. Since these compounds have a low melting point and a low
molecular weight, high solubility of the lithium salt may be
expected. Furthermore, normal butyl normal propyl sulfone, ethyl
normal propyl sulfone and the like are desirable since they have a
low melting point and a higher molecular weight, and thus a high
boiling point and a high flash point may be expected. The kind of
sulfone-based compound used may be one kind or two or more
kinds.
[0034] When the amount of the sulfone-based compound in the
nonaqueous solvent is lower than 50% by volume, oxidation
decomposition of the nonaqueous electrolytic solution by the
positive electrode proceeds, whereby a charge-discharge cycle life
is shorten. On the other hand, when the amount of the sulfone-based
compound in the nonaqueous solvent is more than 95% by volume,
reduction decomposition of the nonaqueous electrolytic solution by
the negative electrode, decrease in output due to decrease in
dissociation of the lithium salt, and increase in the viscosity of
the electrolytic solution are caused, whereby a charge-discharge
cycle life and output are decreased. Accordingly, it is desirable
that the content of the sulfone-based compound in the nonaqueous
solvent is in the range of from 50 to 95% by volume.
[0035] The nonaqueous solvent may include a sultone-based compound
besides the sulfone-based compound represented by Formula 1.
Accordingly, high cycle performance may be obtained and the amount
of gas generation may be decreased. As the sultone-based compound,
various cyclic sultones may be used. Examples of the cyclic
sultone-based compound may include 1,3-propanesultone,
1,4-butanesultone and the like. The kind of sultone-based compound
used may be one kind or two or more kinds.
[0036] It is desirable that the total amount of the sulfone-based
compound and sultone-based compound is 80% by volume or more of the
nonaqueous solvent. Accordingly, a nonaqueous electrolyte secondary
battery that generates a low amount of gas and is excellent in
cycle performance may be realized. This effect may also be obtained
when the total amount of the sulfone-based compound and
sultone-based compound is adjusted to 100% by volume of the
nonaqueous solvent.
[0037] The nonaqueous solvent may include other organic solvent
other than the sulfone-based compound and sultone-based compound.
Examples of the other solvent may include ethylene carbonate (EC),
propylene carbonate (PC), dimethyl carbonate (DMC), methyl ethyl
carbonate (MEC), diethyl carbonate (DEC), .gamma.-butyrolactone
(BL), acetonitrile (AN), ethyl acetate (EA), toluene, xylene,
methyl acetate (MA) and the like. The kind of organic solvent used
may be one kind or two or more kinds. In order to compensate the
low solubility of the lithium salt in the sulfone-based compound,
cyclic carbonates such as EC and PC that have a high dielectric
constant and high lithium salt solubility are desirable. In order
to suppress gas generation and further improve cycle performance,
it is desirable that the nonaqueous solvent comprises the
sulfone-based compound represented by Formula 1 by from 50 to 95%
by volume and the cyclic carbonate by from 5 to 50% by volume (more
preferably from 20 to 50% by volume). When the content of the
cyclic carbonate in the nonaqueous solvent is from 20 to 50% by
volume, it is desirable that the content of the sulfone-based
compound in the nonaqueous solvent is adjusted to from 50 to 80% by
volume. Furthermore, in view of improvement of ion conductivity and
improvement of impregnability by decreasing viscosity, chain
carbonates such as DMC, DEC and MEC are desirable.
[0038] Examples of the lithium salt may include lithium perchlorate
(LiClO.sub.4), lithium hexafluorophosphate (LiPF.sub.6), lithium
tetrafluoroborate (LiBF.sub.4), lithium trifluoromethylsulfonate,
lithium bistrifluoromethylsulfonylimide (LiTFSI), lithium
bispentafluoroethylsulfonylimide and the like. The kind of lithium
salt used may be one kind or two or more kinds. LiPF.sub.6 and
LiBF.sub.4 are desirable since the concentration of the lithium
salt may be increased and better cycle performance may be obtained,
and a mixed salt thereof may also be used.
[0039] The electrolytic solution in the nonaqueous electrolyte
secondary battery may be obtained by cutting a case (for example, a
metal can, a container made of an aluminum-containing laminate
film, or the like), removing a laminate or wound body comprising
electrodes and a separator in which the electrolytic solution has
been soaked, and expressing the solution therefrom. The solution
may be collected by applying a pressure to the above-mentioned
laminate or wound body, or may be collected by centrifugation. The
obtained solution may be separated by fractional distillation by
utilizing difference in boiling points, and the constitutional
compound species may be identified by an NMR method. Alternatively,
the solution may be analyzed by chromatography. In this case, after
identification of the compound species by mass analysis or the
like, the correlationships between the amounts of the compounds and
detection sensitivities are measured and are checked against the
detection sensitivities of an object solution to be analyzed,
whereby the composition of the various compound species in the
object solution may be determined.
(Case)
[0040] Examples of the case may include cans made of metals or
resins, and containers made of laminates. Examples of the metal
cans may include square-shaped containers of aluminum, iron,
stainless and the like. Alternatively, square-shaped containers of
plastics, ceramics and the like may also be used as the case.
Examples of the laminate containers may include one obtained by
combining a metal layer of aluminum, copper, stainless or the like
with a resin layer to give a laminate material, and forming the
laminate material into a sac-like shape by hot melt adhesion. The
laminate container is desirable since generation of gas inside of
the container may be detected as a change in the appearance of the
battery.
[0041] In the initial charging of the nonaqueous electrolyte
secondary battery, gas is generated in the case by the reaction
between the negative electrode and the nonaqueous electrolytic
solution (reduction decomposition of the nonaqueous electrolytic
solution by the negative electrode). The gas generated by the
reaction between the negative electrode and the nonaqueous
electrolytic solution may be removed by removing the gas in the
case after the initial charging, or after further charging and
discharging one or more times after the initial charging and
discharging. According to the first embodiment, the gas generation
by the reaction between the positive electrode and the nonaqueous
electrolytic solution may be decreased. Therefore, by removing the
gas derived from the negative electrode by a removal treatment, the
amount of the gas in the case may further be decreased, whereby
cycle performance may further be improved.
[0042] The specific example of the nonaqueous electrolyte secondary
battery of the first embodiment is shown in FIGS. 1 and 2. FIG. 1
is a view that schematically shows the cross-section obtained by
cutting a flat-type nonaqueous electrolyte secondary battery in the
direction of the thickness of the battery, and FIG. 2 is an
enlarged cross-sectional view showing the portion A of FIG. 1. The
nonaqueous electrolyte secondary battery comprises a case 1 made of
a laminate film, an electrode group 2 that is housed in the case 1,
and a nonaqueous electrolytic solution (not shown). The case 1 made
of a laminate film is obtained by molding a laminate film
comprising a metal layer combined with a resin layer into a
sac-like shape by hot melt adhesion. The electrode group 2
comprises a plurality of sets of a positive electrode 3, a negative
electrode 4 and a separator 5 that provided between the positive
electrode 3 and the negative electrode 4, wherein the sets are
stacked sequentially. The positive electrode 3 comprises a positive
electrode current collector 3a, and a positive electrode material
layer 3b that is held by both sides or one side of the positive
electrode current collector 3a. The negative electrode 4 comprises
a negative electrode current collector 4a, and a negative electrode
material layer 4b that is held by both sides or one side of the
negative electrode current collector 4a. A belt-shaped positive
electrode terminal 6 is electrically connected to the positive
electrode current collector 3a of the positive electrode 3, and the
tip thereof is extended to outside through the hot melt-adhered
part of the case 1. On the other hand, a belt-shaped negative
electrode terminal 7 is electrically connected to the negative
electrode current collector 4a of the negative electrode 4, and the
tip thereof is extended to outside through the hot melt-adhered
part of the case 1.
[0043] Examples of the separator may include polyolefin porous
films of polyethylene, polypropylene and the like, cellulose
nonwoven fabric, polyethylene terephthalate nonwoven fabric, and
polyolefin nonwoven fabric.
[0044] The positive electrode terminal is electrically connected to
the positive electrode, and has a function to electrically bridge
the outside of the battery and the positive electrode. The shape of
the positive electrode terminal is not limited to the belt shape as
shown in FIG. 1, and may have, for example, a ribbon shape or rod
shape. Furthermore, a part of the positive electrode current
collector may be used as the positive electrode terminal, or the
positive electrode terminal may be a part other than the positive
electrode current collector. The positive electrode terminal may be
formed of, for example, aluminum, an aluminum alloy, titanium or
the like.
[0045] The negative electrode terminal is electrically connected to
the negative electrode, and has a function to electrically bridge
the outside of the battery and the negative electrode. The shape of
the negative electrode terminal is not limited to the belt shape as
shown in FIG. 1, and may have, for example, a ribbon shape or rod
shape. Furthermore, a part of the negative electrode current
collector may be used as the negative electrode terminal, or the
negative electrode terminal may be a part other than the negative
electrode current collector. The negative electrode terminal may be
formed of, for example, aluminum, an aluminum alloy, copper,
stainless or the like. Aluminum and an aluminum alloy are desirable
since they are light and excellent in weldability.
[0046] Although FIGS. 1 and 2 show a nonaqueous electrolyte
secondary battery comprising a laminated electrode group and a case
made of a laminate, the form of the electrode group and the kind of
case of the nonaqueous electrolyte secondary battery are not
limited to those shown in the drawings, and any form and kind may
be used as long as they may be used for a nonaqueous electrolyte
secondary battery. For example, a wound type electrode group may be
used, and a metal can may be used for the case.
Second embodiment) The battery pack of the second embodiment
comprises one or a plurality of the nonaqueous electrolyte
secondary battery (unit cell) of the first embodiment. When it has
the unit cells, they are connected with each other in electrically
series or parallel.
[0047] Such battery pack is explained in detail with reference to
FIGS. 3 and 4.
[0048] For example, a flat-type nonaqueous electrolyte secondary
battery may be used for the unit cells. The unit cells 21 that are
constituted by flat-type nonaqueous electrolyte secondary batteries
are stacked so that a positive electrode terminal 16 and a negative
electrode terminal 17 that are extended to outside are aligned in
the same direction, and are bound by an adhesive tape 22 to
constitute a battery module 23. As shown in FIG. 4, the unit cells
21 are connected electrically in series with each other.
[0049] A printed wiring board 24 is disposed opposing to the side
surface of the unit cells 21 from which the negative electrode
terminal 17 and positive electrode terminal 16 are extended. As
shown in FIG. 4, a thermistor 25, a protective circuit 26, and a
terminal 27 for carrying a current to an external device are
mounted on the printed wiring board 24. In addition, an insulating
board (not shown) is attached to the surface of the protective
circuit substrate 24, which faces the battery module 23, so as to
avoid unnecessary connection with the wiring of the battery module
23.
[0050] A positive electrode lead 28 is connected to the positive
electrode terminal 16 that is positioned at the lowermost layer of
the battery module 23, and the tip thereof is inserted to and
electrically connected to a positive electrode connector 29 of the
printed wiring board 24. A negative electrode lead 30 is connected
to the negative electrode terminal 17 that is positioned at the
uppermost layer of the battery module 23, and the tip thereof is
inserted to and electrically connected to a negative electrode
connector 31 of the printed wiring board 24. These connectors 29
and 31 are connected to a protective circuit 26 via wirings 32 and
33 that are formed on the printed wiring board 24.
[0051] The thermistor 25 detects the temperature of the unit cells
21, and the detection signal thereof is sent to the protective
circuit 26. The protective circuit 26 may break a plus wiring 34a
and a minus wiring 34b between the protective circuit 26 and the
terminal 27 for carrying a current to an external device, under a
predetermined condition. The predetermined condition refers to, for
example, the time at which the detection temperature of the
thermistor 25 reaches a predetermined temperature or more.
Furthermore, the predetermined condition refers to the time at
which over-charge, over-discharge, over-current or the like of the
unit cells 21 are detected. The detection of over-charge or the
like is performed in the individual unit cells 21 or the entirety
of the unit cells 21. When detection is performed in the individual
unit cell 21, a battery voltage may be detected, or a positive
electrode potential or negative electrode potential may be
detected. In the latter case, a lithium electrode that is used as a
reference electrode is inserted in the individual unit cell 21. In
the case of FIGS. 3 and 4, wirings 35 for detection of a voltage
are connected to the respective unit cells 21, and detection
signals are sent to the protective circuit 26 via the wirings
35.
[0052] Protective sheets 36 made of a rubber or resin are disposed
respectively on the three side surfaces of the battery module 23
except for the side surface from which the positive electrode
terminal 16 and negative electrode terminal 17 protrude.
[0053] The battery module 23 is housed in a housing container 37
together with the respective protective sheets 36 and the printed
wiring board 24. Namely, the protective sheets 36 are disposed
respectively on the both inner surfaces in the longitudinal side
direction and the inner surface in the short side direction of the
housing container 37, and the printed wiring board 24 is disposed
on the inner surface on the opposite side in the short side
direction. The battery module 23 is positioned in a space
surrounded by the protective sheets 36 and the printed wiring board
24. A lid 38 is attached to the upper surface of the housing
container 37.
[0054] Alternatively, the battery module 23 may be fixed by using a
heat shrink tape instead of the adhesive tape 22. In this case, the
protective sheets are disposed on both side surfaces of the battery
module, the battery module is wound around a heat shrink tube, and
the heat shrink tube is shrank by heating to bind the battery
module.
[0055] Although an embodiment in which the unit cells 21 are
connected with each other in series is shown in FIGS. 3 and 4, the
unit cells may be connected with each other in parallel so as to
increase a battery capacity. Alternatively, assembled battery packs
may be connected with each other in series or parallel.
[0056] Furthermore, the embodiment of the battery pack is suitably
changed according to use. Preferable use of the battery pack is one
for which cycle performance at high currents is desired. Specific
examples may include uses in power sources for digital cameras, and
in-car uses in two to four-wheeled hybrid battery automobiles, two
to four-wheeled battery automobiles, motor assisted bicycles and
the like. In-car uses are preferable.
EXAMPLES
[0057] Hereinafter, the Examples of the embodiments are explained
in detail with reference to the drawings. In the following
Examples, the battery structure shown in FIG. 1 was adopted.
Example 1
[0058] A slurry was prepared by kneading 90% by weight of
LiMn.sub.1.5Ni.sub.0.5O.sub.4 powder as positive electrode active
material, 2% by weight of acetylene black and 5% by weight of
graphite, 5% by weight of polyvinylidene fluoride as a binder, and
N-methylpyrrolidone as a solvent. The obtained slurry was applied
on the both surfaces of an aluminum foil having a thickness of 15
.mu.m as a positive electrode current collector. At that time, the
slurry was not applied on the part at 5 mm from one long side edge
of the positive electrode current collector, whereby an unapplied
part was formed. The current collector on which the slurry had been
applied was subsequently dried and pressed to prepare a positive
electrode sheet having a width of 69 mm and a length of 93 mm.
Aluminum ribbons each having a width of 5 mm and a thickness of 0.1
mm were weld on the three parts on the unapplied part of the
positive electrode sheet to form positive electrode tabs.
[0059] A slurry was prepared by adding 90% by weight of
Li.sub.4Ti.sub.5O.sub.12 powder as a negative electrode active
material and 5% by weight of artificial graphite as electron
conductive substance and 5% by weight of polyvinylidene fluoride
(PVdF) to an N-methylpyrrolidone (NMP) solution and mixing them.
The obtained slurry was applied on both surfaces of an aluminum
foil having a thickness of 25 .mu.m as a negative electrode current
collector. At that time, the slurry was not applied on a part at 5
mm from one longitudinal side edge of the negative electrode
current collector, whereby an unapplied part was formed. The
current collector on which the slurry had been applied was
subsequently dried and pressed to give a negative electrode sheet.
The obtained negative electrode sheet was cut into a width 70 mm
and a length of 91 mm so that the unapplied portion was present on
the part at 5 mm from one longitudinal side edge. Aluminum ribbons
each having a width of 5 mm and a thickness of 0.1 mm were weld on
the three parts on the unapplied part to form negative electrode
tabs.
[0060] A polyethylene porous film having a thickness of 30 .mu.m
and a width of 72 mm was used as a separator. The belt-shaped
positive electrode sheet, separator, belt-shaped negative electrode
sheet and separator were stacked 25 times, each in this order, to
prepare an electrode group. Three positive electrode tabs were
superposed, and weld on a positive electrode terminal composed of
an aluminum sheet having a thickness of 0.1 mm, a width of 30 mm
and a length of 50 mm. Three negative electrode tabs were
superposed, and weld on a negative electrode terminal composed of
an aluminum sheet having a thickness of 0.1 mm, a width of 30 mm
and a length of 50 mm.
[0061] The electrode group was housed in a case made of an
aluminum-containing laminate film. 1 M of LiBF.sub.4 was dissolved
in a nonaqueous solvent obtained by mixing ethyl isopropyl sulfone
(EIPS) and propylene carbonate (PC) at a volume ratio of 1:1 to
give a nonaqueous electrolytic solution. The amount of the
sulfone-based compound in the nonaqueous solvent was 50% by volume.
The nonaqueous electrolytic solution in an amount of such a degree
that the entirety of the electrode group is soaked was injected to
the electrode group in the case, and the case was sealed by heat
sealing to prepare a nonaqueous electrolyte secondary battery.
Example 2
[0062] A nonaqueous electrolyte secondary battery was prepared in a
similar manner to Example 1, except that diethylcarbonate (DEC) was
used instead of PC.
Example 3
[0063] A nonaqueous electrolyte secondary battery was prepared in a
similar manner to Example 1, except that isopropyl methyl sulfone
(IPMS) was used instead of SIPS.
Example 4
[0064] A nonaqueous electrolyte secondary battery was prepared in a
similar manner to Example 1, except that the mixing volume ratio of
EIPS:PC was adjusted to 95:5 and the amount of the sulfone-based
compound in the nonaqueous solvent was adjusted to 95% by
volume.
Example 5
[0065] A nonaqueous electrolyte secondary battery was prepared in a
similar manner to Example 1, except that SIPS, propanesultone (PS)
and PC were mixed by a volume ratio of 90:5:5, the amount of the
sulfone-based compound in the nonaqueous solvent was adjusted to
90% by volume, and the total amount of the sulfone-based compound
and sultone-based compound was adjusted to 95% by volume.
Comparative Example 1
[0066] A nonaqueous electrolyte secondary battery was prepared in a
similar manner to Example 1, except that a nonaqueous electrolytic
solution obtained by dissolving 1 M of LiPF.sub.6 in a mixture of
ethylene carbonate (EC) and diethyl carbonate (DEC) at a volume
ratio of 1:2.
Comparative Example 2
[0067] A nonaqueous electrolyte secondary battery was prepared in a
similar manner to Example 1, except that a nonaqueous electrolytic
solution obtained by dissolving 1 M of LiBF.sub.4 in only EIPS was
used.
Comparative Example 3
[0068] Dimethylsulfone (DES) and PC were mixed at a volume ratio
1:1 on a hot plate of 60.degree. C. 1 M of LiBF.sub.4 was dissolved
in this solution. DEC has a solid at an normal temperatures and
R.sub.1 and R.sub.2 in Formula 1 both being methyl groups. However,
a white solid was precipitated when the temperature was decreased
to room temperature, and the solution was separated into a liquid
layer and the precipitated solid and could not be used as a
nonaqueous electrolytic solution. Accordingly, an evaluation may
not be performed.
[0069] The nonaqueous electrolyte secondary batteries obtained in
Examples 1 to 5 and Comparative Examples 1 and 2 were each constant
current constant voltage charged to 3.3 V at 1.5 A to attainment of
30 mV, and discharged to 2.7 V at 1.5 A. This was repeated twice,
the battery was stored at 45.degree. C. for 40 hours, the gas in
the case was removed by vacuum, and the case was sealed again.
Thereafter charging and discharging were performed 100 times under
the same condition as mentioned above. The relationships between
the number of the charge-discharge cycles and the discharge
capacity maintenance rate are shown in FIG. 5.
[0070] As is apparent from FIG. 5, according to Examples 1 to 5, it
is found that the decrease in the discharge capacity maintenance
rate in accordance with the progress of the charge-discharge cycles
is moderate as compared to Comparative Examples 1 and 2, and higher
cycle performance may be obtained. By comparing Examples 1 and 2,
it is found that the cycle performance was more excellent in the
case when the cyclic carbonate was used (Example 1) than in the
case when DEC was used (Example 2). Furthermore, by comparing
Examples 1 and 3, it is found that more excellent cycle performance
may be obtained in the case when EIPS was used as the sultone-based
compound than in the case when IPMS was used.
[0071] According to the embodiments and Examples as explained
above, the cycle performance of the nonaqueous electrolyte
secondary battery may be improved.
[0072] While certain embodiments have been described, these
embodiments have been presented by way of example only, and are not
intended to limit the scope of the inventions. Indeed, the novel
embodiments described herein may be embodied in a variety of other
forms; furthermore, various omissions, substitutions and changes in
the form of the embodiments described herein may be made without
departing from the spirit of the inventions. The accompanying
claims and their equivalents are intended to cover such forms or
modifications as would fall within the scope and spirit of the
inventions.
* * * * *